Porcelain Glaze Crazing: Causes, Safety and Care
Crazing is a network of fine cracks in the glaze of a ceramic piece. It appears when the glaze is under tension because it shrank more than the clay body underneath as the piece cooled. Sometimes it is a flaw. Sometimes it is a deliberate decorative effect. Whether a crazed piece is a problem depends on what it is used for, and that is what this guide helps you work out.
TL;DR
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Crazing means the glaze contracted more than the body during cooling, so it cracks into a fine web. The National Park Service describes it this way.
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It can show up right out of the kiln or months later, and the same glaze may craze on one clay body and not on another.
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Crazing alone does not prove a piece is unsafe, but lead content is a separate question that only a reliable source or a test can answer.
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Makers fix crazing by lowering the glaze’s expansion and testing one change at a time. Owners should hand wash gently and keep crazed pieces away from sudden temperature swings.
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Crackle glaze is crazing used on purpose, and it is a classic look in Asian porcelain.
Table of Contents
What crazing means
The word comes from the way the surface looks: a spiderweb or crackled-eggshell pattern of hairline cracks that run through the glaze layer. The Conservation Wiki and museum guides use the same basic definition. Crazing is cracking of the glaze surface caused by tension between the glaze and the body.
Think of glaze as a thin layer of glass fused to the clay. Both materials expand when hot and shrink when cooling, but not by the same amount. If the glaze shrinks more, it ends up stretched over a body that is smaller than it wants to be. Glass handles being squeezed far better than being stretched, so it gives way in a network of fine cracks. The technical term for how well the two match is glaze fit, and it is the difference between the thermal expansion of the fired body and the glaze. Crazing happens when the tension in the glaze exceeds what the glaze can withstand.
A few signs help you spot it:
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A web of hairline lines across the glaze, often following a polygon-like pattern.
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Lines that catch tea, coffee, or dust and look darker than the surrounding glaze.
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Sometimes a faint ticking or pinging sound as it develops. Glaze that is too small for the body commonly crazes after cooling, and soft pinging sounds may accompany it.
Why porcelain glaze crazes
The root cause is almost always a mismatch of expansion between glaze and body. One technical reference calls a glaze/body expansion mismatch the dominant cause: when a fired glaze expands significantly more than the body, it will craze. But several other factors push a piece over the edge.
Glaze chemistry
Sodium and potassium fluxes, which help glaze melt, have high expansion. Silica and alumina have lower expansion. A glaze rich in the first group and short on the second is more likely to craze. That is why so many repair recipes revolve around adjusting those ingredients.
The clay body
The same glaze recipe can behave differently from one body to the next. Crazing can happen immediately after firing or later, and a recipe may craze on one clay and not on another. This is why a glaze that worked in one studio can craze when moved to a different clay.
Firing and cooling
The National Park Service notes that overfiring or rapid cooling can also contribute to crazing. Firing technique helps at the margins, but a pottery-technology reference points out that firing alone cannot reliably prevent crazing when glaze and body are fundamentally incompatible. The fix has to start with the fit.
Delayed crazing
Pieces sometimes craze weeks, months or even years after they leave the kiln. Marginal glaze fit is usually the reason: the piece starts out only slightly stressed, and moisture absorption into the body, temperature changes or simple aging nudge it past the limit. If you buy porcelain and see fine lines appear later, this is the usual story.
Crazing versus other glaze and body defects
Not every flaw on a glazed piece is crazing. Telling them apart matters because the causes and the consequences differ.
| Defect | What you see | What is happening |
|---|---|---|
| Crazing | Fine web of hairline cracks in the glaze | Glaze is under tension, having contracted more than the body |
| Shivering | Glaze flakes or peels away, usually at rims and edges | Glaze is under excessive compression |
| Crawling | Glaze pulls into islands, exposing bare clay | Often glaze applied over a dirty or greasy surface |
| Pitting | Pinholes to larger spots | Gas or surface issues during firing |
| Blistering | Easily crushed bumps | Trapped gas pushing the glaze up |
A crack in the porcelain itself looks different from crazing. A structural crack is usually a single line or a few lines that run through the body and often through the whole wall of the piece. Crazing stays in the glaze skin and forms a dense mesh. If a line goes through the wall, or the piece rings dull when you tap it, treat it as damage rather than crazing and stop using it for food or liquid.
Crackle glaze: when crazing is the point
Crackle is crazing used as decoration. The National Park Service notes that crackle may be an intentional decorative technique. Potters deliberately choose a glaze and body combination with a mismatch, so the network of lines forms evenly and attractively. Some makers rub ink or tea into the cracks to make the pattern stand out.
The difference between a deliberate crackle and accidental crazing comes down to control. Intentional crackle is even, consistent from piece to piece and planned for the intended use. Accidental crazing tends to be patchy, may appear on only some pieces in a batch and often develops over time.
If you like the look, our cracked glaze Korean style noodle soup bowl with lid is an example of how a crackled surface can be part of a design instead of a defect.
Is crazed porcelain safe for food and drink?
This is the question behind most searches, and the honest answer has two parts: bacteria and lead.
Bacteria and hygiene
The cracks are tiny, and they can hold onto residue. In one study discussed in a ceramics trade publication, crazed surfaces retained bacteria after rinsing with water and with warm soapy water, but dishwasher washing removed or killed detectable bacteria. Non-crazed surfaces could also retain bacteria. The takeaway from that discussion is that heat, soap and water remove bacteria even from crazed surfaces, so crazing alone is not proof of unsafe foodware. In practice, wash crazed mugs and bowls thoroughly and dry them completely, and be more careful with them than with a flawless piece.
Lead and glaze quality
Lead is the bigger safety question, and it has nothing to do with whether a piece is crazed. The FDA warns that improperly manufactured or fired lead-glazed pottery may contaminate food and drink. It advises against using pottery for cooking, serving or storing food or drinks when a lead test is positive, the source is unreliable, or a decorative-only warning label is present. Washing or boiling cannot remove lead from pottery.
The FDA’s pottery guidance also sets limits by product type, including 0.5 micrograms of lead per milliliter of leaching solution for cups, mugs and pitchers. Those are the limits that matter for the cup you drink from.
A practical approach:
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Check whether the piece was sold as food-safe by a maker or seller you can ask. If you cannot find out, use it for display.
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Be especially cautious with older, damaged or unmarked pieces. A university museum advises checking ceramics for lead or uranium glazes before eating or drinking from them, especially older or damaged items.
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If you are pregnant, or the piece will be used by a young child, ask your pediatrician or doctor about lead exposure before using any pottery of uncertain origin for hot drinks.
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Use a piece with doubtful provenance for flowers, candles or display instead of beverages.
Strength and leakage
Crazing also affects durability. It greatly weakens glazed ware, and in earthenware it can cause leakage. Porcelain is vitrified and far less porous than earthenware, so leakage is less of a worry, but a crazed piece is still more fragile than a sound one. For tile, the National Park Service says crazing is not generally a serious failure unless it visibly extends into the porous body, but dirt that enters the cracks cannot be removed and will discolor the piece. That staining is the same thing you see in a crazed mug that has absorbed tea over time.
How makers prevent crazing
If you make pottery, the goal is to bring the glaze’s expansion closer to the body’s. The fixes are chemistry first and firing second.
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Lower the glaze expansion. Adding lower-expansion materials such as boron, silica, clay or talc, or reducing high-expansion materials such as feldspar, is a standard correction. The same source recommends changing one material at a time and testing.
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Reduce sodium and potassium fluxes. Lowering these high-expansion fluxes and adding silica or alumina is another common route.
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Check the body. The same recipe can fit one clay and not another, so test on the body you actually use.
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Apply a thinner coat. Thick glaze layers carry more stress.
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Control cooling. Slower, controlled cooling may help, though it will not rescue a bad fit.
One caution about the most common advice: simply adding silica is not always a clean solution. It can change how the glaze looks, shifting gloss, color and melt, so test small batches before committing a kiln load.
Test before you trust. Fire sample tiles or small pieces, let them cool fully, and look again after a few days or weeks, since delayed crazing is common. Keep notes on every change so you can trace the cause.
Caring for porcelain with crazed glaze
If you already own a crazed piece, or a crackle-glazed one you love, gentle care keeps it looking good and prevents further stress.
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Dust regularly with a soft brush. Museum conservators recommend regular soft-brush dusting for ceramics.
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Hand wash with water and a gentle detergent, and avoid harsh chemicals. The same guidance covers this.
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Avoid drastic temperature changes. Do not pour boiling water into a cold crazed cup, and do not move a piece straight from freezer to hot oven. Rapid changes stress the glaze.
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Dry thoroughly so moisture does not sit in the cracks.
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Do not soak crazed pieces for long periods.
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Leave repairs to professionals. Glue and fillers on food surfaces are not a safe do-it-yourself job.
If staining has already made the cracks visible and you dislike it, remember that the dirt in the cracks generally cannot be removed. Many people choose to embrace that aged look, or retire the piece to display duty.
Our perspective
We design every piece in-house and produce it by hand, using traditional techniques rooted in Jingdezhen’s porcelain heritage. That is why we think about glaze fit so carefully: the glaze and the body have to be made for each other, and no shortcut replaces testing.
To us, crazing is not a single good-or-bad question. A deliberate crackle can be one of the most beautiful finishes in porcelain, with a surface that tells a story of the kiln. An accidental web of fine lines on a piece meant for daily use is a different matter. Know which one you are looking at, and choose accordingly.
When you shop for handcrafted porcelain, look for makers who talk plainly about how a piece is meant to be used, whether it is for the table or for display, and who stand behind their design process. Small production runs and limited quantities let a studio watch each firing closely instead of trusting a formula and hoping.
Porcelain rewards patience. If you take the time to choose a piece with a glaze you understand, and then care for it gently, it can sit in your home for a very long time.
Alan
Handcrafted porcelain for your home
If you are looking for porcelain with a considered glaze, whether a smooth finish or a crackle you chose on purpose, browse our handcrafted porcelain and ceramic collection. We design each piece in-house and make it by hand, from tealight holders and vases to cups, bowls and plates.
For everyday drinking, start with our handcrafted tea cups and mugs. For display, our vase collection is a good place to look. If you want to understand how glazes and firing shape a finished piece, our guide to the porcelain handmaking process walks through it, and our article on underglaze versus overglaze explains how decoration sits in or on the glaze. To see how porcelain differs from other ceramics, read porcelain versus ceramic home décor.
FAQ
What does crazing mean in ceramics?
Crazing is a fine network of cracks in a ceramic glaze. It happens when the glaze shrinks more than the clay body as the piece cools, leaving the glaze under tension. The result looks like a spiderweb across the surface.
How do you stop glaze from crazing?
The fix is to lower the glaze’s thermal expansion so it matches the body better. Makers typically add silica, alumina, boron, clay or talc, or reduce feldspar and high-expansion fluxes, changing one thing at a time and testing. A thinner coat and controlled cooling can help, but they cannot fix a basic mismatch.
Is it safe to use a mug with crazing?
Crazing by itself does not make a mug unsafe, and thorough washing with heat and soap removes bacteria, with dishwasher washing performing best in the study cited above. The real question is lead. If you do not know where the mug came from or whether its glaze is food-safe, do not drink from it, and ask a reliable source or use a lead test.
Can you repair crazed glaze?
Not in a way that restores the original piece. Once glaze has crazed, the cracks cannot be sealed safely for food use at home. Makers can refire or reglaze a new piece with a corrected recipe, and for valuable items a professional conservator should handle any repair.
Why did my porcelain craze months after I bought it?
Delayed crazing is common when the glaze fit is marginal. Moisture, temperature changes and time can push a slightly stressed glaze past its limit. It is a sign of glaze and body mismatch, not of how you treated the piece.
Sources
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Appendix P: Curatorial Care of Ceramic, Glass, and Stone Objects (National Park Service)
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Preservation Brief 40: Preserving Historic Ceramic Tile Floors (National Park Service)
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Questions and Answers on Lead-Glazed Traditional Pottery (FDA)
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CPG Sec. 545.450 Pottery (Ceramics); Import and Domestic, Lead Contamination (FDA)